24
CHR. P. RAVEN
however, that our previous interpretation of these results is not
satisfactory. Verdonk has made a detailed study of the cell lineage in
normal and lithium-treated embryos of Limnaea. It appeared that the
expected distortion of the head pattern after lithium treatment, the
various primordia being outlined in the 'wrong' places from the start,
did not occur at all. The cephalic plates in cyclocephalic embryos are not
fused in the mid-line, as originally assumed, but connected by a bridge
of small cells behind the apical plate (Fig. 8B). The latter is not reduced,
but may even consist of more than the normal seven cells. Up to a
certain stage, the cleavage mosaic in normal and lithium-treated
embryos is identical. The first visible deviation of development is a
change in the direction of the cleavage spindle in one particular cell,
situated at the base of the dorsal (D) arm of the cross. Its daughter cells
then divide repeatedly, so giving rise to the transverse band of small
cells connecting the cephalic plates, contrary to their behaviour in
normal development, during which they divide no more and take part in
the formation of the head vesicle. Similar aberrations may occur at
about the same stage in the lateral (A and C) arms of the cross, some
blastomeres undergoing more divisions, other cells fewer divisions than
in normal development. After heat-shock treatment the primary effects
are of a similar nature, but located at other places.
These results can hardly be explained on the basis of a mere gradientfield hypothesis. The very localized effects of the treatments, sometimes
being confined to one or a few blastomeres, argue in favour of equally
local variations in the cortical field. We therefore have come to the
conclusion that the morphogenetic field in the cortex has a mosaic
character, in the sense that, besides local variations of a continuous
nature, discontinuous variations of the structure and properties of the
cortex must also be assumed. During cleavage this mosaic is apportioned
between the cells, and plays a part in their further development.
A renewed study of ooplasmic segregation in early stages of Limnaea
has yielded corroborating evidence for such a view. It appeared that
the vegetal pole plasm in eggs fixed immediately after oviposition is
situated somewhat obliquely with respect to the axis of the first
maturation spindle. Moreover, in the equatorial zone of such eggs there
are immediately beneath the cortex several small lenticular patches of
cytoplasm with about the same staining properties as the vegetal pole
plasm (Fig. IB). Generally there are six of these patches. They show no
regular spacing around the equator, but are asymmetrically distributed :
four or five of them are situated near together on that side of the egg
where the boundary of the vegetal pole plasm is highest, one or two on
the opposite side (Raven, 1963) (Fig. 9).
In a fully grown oocyte in the gonad neither the vegetal pole plasm
CHR. P. RAVEN
however, that our previous interpretation of these results is not
satisfactory. Verdonk has made a detailed study of the cell lineage in
normal and lithium-treated embryos of Limnaea. It appeared that the
expected distortion of the head pattern after lithium treatment, the
various primordia being outlined in the 'wrong' places from the start,
did not occur at all. The cephalic plates in cyclocephalic embryos are not
fused in the mid-line, as originally assumed, but connected by a bridge
of small cells behind the apical plate (Fig. 8B). The latter is not reduced,
but may even consist of more than the normal seven cells. Up to a
certain stage, the cleavage mosaic in normal and lithium-treated
embryos is identical. The first visible deviation of development is a
change in the direction of the cleavage spindle in one particular cell,
situated at the base of the dorsal (D) arm of the cross. Its daughter cells
then divide repeatedly, so giving rise to the transverse band of small
cells connecting the cephalic plates, contrary to their behaviour in
normal development, during which they divide no more and take part in
the formation of the head vesicle. Similar aberrations may occur at
about the same stage in the lateral (A and C) arms of the cross, some
blastomeres undergoing more divisions, other cells fewer divisions than
in normal development. After heat-shock treatment the primary effects
are of a similar nature, but located at other places.
These results can hardly be explained on the basis of a mere gradientfield hypothesis. The very localized effects of the treatments, sometimes
being confined to one or a few blastomeres, argue in favour of equally
local variations in the cortical field. We therefore have come to the
conclusion that the morphogenetic field in the cortex has a mosaic
character, in the sense that, besides local variations of a continuous
nature, discontinuous variations of the structure and properties of the
cortex must also be assumed. During cleavage this mosaic is apportioned
between the cells, and plays a part in their further development.
A renewed study of ooplasmic segregation in early stages of Limnaea
has yielded corroborating evidence for such a view. It appeared that
the vegetal pole plasm in eggs fixed immediately after oviposition is
situated somewhat obliquely with respect to the axis of the first
maturation spindle. Moreover, in the equatorial zone of such eggs there
are immediately beneath the cortex several small lenticular patches of
cytoplasm with about the same staining properties as the vegetal pole
plasm (Fig. IB). Generally there are six of these patches. They show no
regular spacing around the equator, but are asymmetrically distributed :
four or five of them are situated near together on that side of the egg
where the boundary of the vegetal pole plasm is highest, one or two on
the opposite side (Raven, 1963) (Fig. 9).
In a fully grown oocyte in the gonad neither the vegetal pole plasm